As an insulin syringe and the like for administering a small amount of a medical agent to a patient, a syringe having a piercing needle fixed to the distal end of an outer cylinder is used. In constructing a prefilled syringe in which a medical agent is filled in advance by using a syringe of this type, it is necessary to seal the tip of the needle. A cap capable of sealing the tip of the needle is proposed, as disclosed in Japanese translation of PCT International Application Publication No. 2010-534546 (patent document 1) and U.S. Pat. No. 6,719,732 (patent document 2).